Full visual function depends on the coordinated action of vision and eye movements to direct our line of sight so that an object of interest will be imaged on the fovea of each eye. To achieve maximum visual acuity and depth perception, we must be able to hold our eyes steady on a stationary or moving target. If normal innervation fails to develop or is compromised by injury, infantile or acquired misalignment of the eyes (strabismus) could result. In fact, at least 3% of children born in the United States are diagnosed with strabismus every year. Early and appropriate treatment of strabismus is essential to prevent loss of visual function. However, treatment of strabismus remains challenging, partly because we lack a definitive understanding of the etiology of the disease. It is likely that misalignment of the eyes in some strabismics is due to improperly calibrated tonic innervation of individual eye muscles or muscle compartments. Ultimately, eye alignment, gaze- holding and eye movements depend on the quality of oculomotor innervation to different extra-ocular muscles (EOM) and muscle compartments. Recent anatomical work, including some from our respective laboratories, shows that there are at least two separate classes of motoneurons that innervate orbital and global layers of EOM. One class (e.g., A-groups) provides en-plaque endings to singly innervate EOM fibers (SIFs). The other class (e.g., C- and S-groups) provides en-grape endings to multiply innervate EOM fibers (MIFs). Importantly, there are clear differences in the sources of signals in pre-motor structures, which modulate these different classes of oculomotor neurons. Our studies will compare and contrast the functional organization and relative roles of different classes of motoneurons (C- and A-groups) that primarily drive either orbital or global muscle fibers. We will test the hypothesis that proper eye alignment could be achieved by the action of different muscle compartments, which have direct insertion on the eye (global layer) or act indirectly by inserting on the recently discovered extraocular muscle pulleys. We will test the hypothesis that C-group motoneurons play a major role in maintaining eye alignment, gaze-holding and vergence. We will accomplish this by conducting neurophysiological studies in monkeys trained to fixate and track moving targets. We will also conduct complimentary neuroanatomical studies to refine our understanding of some of the premotor sources of signals to these different classes of motor neurons. Completion of our studies could significantly advance our understanding of normal and pathological eye alignment, gaze-holding and eye movements. PUBLIC HEALTH RELEVANCE: Treatment and cure of developmental or acquired strabismus requires advancing our knowledge about how the brain controls eye alignment, gaze-holding and eye movements. Our studies will examine the relative roles of different classes of oculomotor neurons that innervate different eye muscle compartments in the above functions.